The invention concerns a process for the extraction of thick coal seams by means of which coal deposits in areas where single-slice (single-back) coal winning is not possible and/or peripheral coal seams can be extracted with high productivity and under improved mine safety conditions.
There are several known technical solutions to the problem of exploitation of coal seams which cannot be extracted in single-section "one-slice" working by the application of hitherto known methods of winning and supporting system, due to geological or other conditions such as previous mining operations, "old-man" (depleted) cavities. Such solutions include inter alia:
(a) Working in an ascending direction at a plurality of slices or sections, while employing stowing. The extracted coal of successive slices advances across the backfilling or stowing of the preceding slices.
(b) Working in a descending direction at a plurality of slices with stowing and/or caving. In the couse of stowing the stowage material, or at least its bottom layer, is stabilized prior to the start of a new slice, or an artificial roof is constructed, or a thin protective layer, and a so-called "coal skin" is left in the roof. In given cases these methods may be combined. If caving is applied, there must be sufficient waiting for the appropriate agglomeration of the goaf (stowage) of the preceding section. If an artificial roof is formed e.g. a net or mesh is laid down before caving is carried out, in the most advanced method, the goaf, or at least the bottom layer thereof is rendered solid. Again, if need be, the known methods are combined. Extraction of the slice situated below the caving can only be commenced under the caving if the goaf has been suitably agglomerated and/or rendered solid.
(c) Also known is a process wherein the thick seam is divided into fewer working slices than the number of slices derived from the practical limit in height at which the winning and support may be used; or it may even be extracted in one slice only by the caving located in the roof of the extraction space. The winning of coal or other useful minerals located in the roof by caving i.e. by block caving (applying the nomenclature used principally in ore mining) can also be carried out by different methods.
(d) In one of the known methods of block caving, the in situ caving of the required quantity of useful minerals at a given time is excited by blasting or by other means. Such a method is described e.g. in Hungarian patent specification No. 153,410 and in Soviet patent specification No. 473,829.
(e) Similarly, caving is provoked and promoted by external intervention, by displacing the rear elements of supporting units of varying systems or by using individual manipulating devices built for that purpose. A technical solution of this type is disclosed, inter alia, in Sovient patent specification No. 480,846.
(f) Block caving combined with the formation of coal pillars (coal ribs) is often applied in random seams cut through by faults and at the periphery of broad or longwall seams employing fully mechanized mining, working as a cleaning-up extraction operation. Except for seams of relatively small dimensions, at such seams a plurality of narrow faced working faces has to be established, in accordance with the prevailing conditions. Between the extraction faces coal pillars (legs) which are left behind, are left temporarily or are frequently permanently left in and are therefore lost, according to the technology applied. This residual coal pillar increases the quantity of lost coal, particularly if it has low strength. However, where in order to reduce the loss, the pillar dimensions are chosen to be smaller, the pillar may crumble and hence cannot perform its function, on the contrary, this significantly increases the fire hazard.
(g) Soviet patent specification No. 473,829 entitled "A METHOD of extraction of THICK SEAMS" describes a method of block caving wherein the winning of coal is carried out by drilling and blasting or by some other mechanical comminution. In order to carry out this process with the requisite safety and control, in the interests of assuring the firmness of the roof pillar, a suitable inclination in the direction of the virgin coal seam must be given to the face of the roof.
According to this Soviet patent specification during the development of the extraction space a pressure zone of high concentration is formed, as a consequence of which it becomes uncontrollable and a breakage (fall) extending over the totality of the thickness of the coal seam can occur.
If, on the other hand, the process according to the Soviet invention is applied, the roof to be extracted by drilling and blasting or by other mechanical means can be relieved from the pressure of the pillar and it will retain its roof strength during the total period of the winning cycle.
Although the known methods are widely used in given circumstances, they cannot meet the complex requirements of highly concentrated production and increased productivity according to modern concepts, combined with the indispensably requisite high degree of mine safety. In any case they all have various drawbacks that cannot be obviated completely but can only be restricted. Such drawbacks include, inter alia:
They cannot satisfactorily secure safety conditions in the mines and in mining, particularly the prevention of the hazards of falls, gas and fire. This is true e.g. of the less advanced methods of (b), (d), (e), (f) and (g); PA1 Production can only be performed at low intensity, e.g. certain variants of (a), (b), (d) and (e); PA1 Poor level of economy, e.g. (a), (d) and (f). PA1 Their domain of application greatly depends on the dimensions and geological conditions of the seam; PA1 Generally the given method of extraction prescribes almost compulsorily the application of predetermined supports and extraction machinery; PA1 Complicated, expensive machinery and technology is required, e.g. certain variants of (b), (d), (e) and (f);